1. Field of the Invention
This invention relates to a hybrid electric vehicle and more particularly, to a hybrid electric vehicle having a variable displacement engine which provides for improved fuel economy and load performance.
2. Background Art
Various types of automotive vehicles have been designed and manufactured for the purpose of improving fuel economy. One type of vehicle, commonly known as a hybrid electric vehicle (“HEV”), utilizes both an internal combustion engine and one or more electric motors to generate power and torque. The electric motor(s) within a hybrid electric vehicle provides the vehicle with additional degrees of freedom in delivering the driver-demanded torque. Particularly, hybrid electric vehicles may have the flexibility of using the electric motor(s) as the sole source of torque at low load operating conditions. Additionally, at relatively high loads, a hybrid electric vehicle can employ or activate both the internal combustion engine and the electric motor(s) to supply the driver-demanded torque. Due to the ability of the electric motor(s) to augment the engine generated torque at high load operating conditions, hybrid electric vehicles typically employ relatively small or “downsized” internal combustion engines, which provide for improved fuel economy.
One drawback associated with these hybrid electric type vehicles is that the electric motors used within these vehicles receive power from an electrical energy storage device (e.g., a battery) which becomes depleted over time. When the electrical energy storage device or battery becomes depleted, the motors are no longer able to supplement the engine to deliver relatively high amounts of torque. This especially presents a problem in steady state high load situations (e.g., when towing or hauling heavy cargo), as the propulsion system (e.g., the engine and electric motor) may not be able to deliver the desired or demanded torque consistently and/or over an extended period of time. Thus, when high load steady state performance is required within a vehicle (e.g., within large trucks, sport-utility vehicles, and other vehicles that are often used for towing or hauling), the vehicle's engine cannot be “downsized” and the hybridization-related fuel economy benefits cannot realized to the maximum potential.
Another type of vehicle, known as a variable displacement engine (“VDE”) type vehicle, conserves fuel by selectively disabling or deactivating some of the engine's cylinders under certain operating conditions. Particularly, during low engine load operation, a VDE type vehicle conserves fuel by operating on only some of the engine's cylinders (e.g., on four, five, six, or seven of eight cylinders). During relatively high speeds and/or high load operation, a VDE type vehicle can meet the driver-demanded speed and/or torque by operating on all of the engine's cylinders (e.g., on eight of eight cylinders). While these VDE type vehicles improve fuel economy and provide the torque required to satisfy relatively high load operation, they suffer from some drawbacks.
For example and without limitation, during relatively low and/or transient load operation (e.g., when the operating load varies significantly over a short period of time), the power train of a VDE type vehicle frequently shifts between various cylinder operating modes (e.g., the engine shifts between four, six, eight and ten cylinder operation). This frequent shifting between cylinder operating modes results in noise and vibration harshness (“NVH”) problems which are perceivable by the driver and which give the vehicle a “rough”, “inconsistent” and/or “non-responsive” feel or ride.
There is therefore a need for a vehicle which provides an improved fuel economy, which provides steady state high load performance, and which operates smoothly and responsively under transient load conditions.